The invention relates to a method of regenerating used hydrocarbon conversion catalyst by the combustion of coke on the catalyst in a fluidized combustion zone. This invention specifically relates to a process for the conversion of heavy hydrocarbons into lighter hydrocarbons with a fluidized stream of catalyst particles and regeneration of the catalyst particles to remove coke which acts to deactivate the catalyst.
Fluidized catalytic cracking is a hydrocarbon conversion process accomplished by contacting hydrocarbons in a fluidized reaction zone with a catalyst composed of finely divided particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds substantial amounts of highly carbonaceous material referred to as coke is deposited on the catalyst. A high temperature regeneration within a regeneration zone operation burns coke from the catalyst. Coke-containing catalyst, referred to herein as spent catalyst, is continually removed from the reaction zone and replaced by essentially coke free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and regeneration zone. Methods for cracking hydrocarbons in a fluidized stream of catalyst, transporting catalyst between reaction and regeneration zones, and combusting coke in the regenerator are well known by those skilled in the art of fluidized catalytic cracking (FCC) processes. To this end the art is replete with vessel configurations for contacting catalyst particles with feed and regeneration gas respectively.
A common objective of these configurations is maximizing product yield from the reactor while minimizing operating and equipment costs. Optimization of feedstock conversion ordinarily requires essentially complete removal of coke from the catalyst. This essentially complete removal of coke from catalyst is often referred to as complete regeneration. Complete regeneration produces a catalyst having less than 0.1 and preferably less than 0.05 weight percent coke. In order to obtain complete regeneration, oxygen in excess of the stoichiometeric amount necessary for the combustion of coke to carbon oxides is charged to the regenerator. Excess oxygen in the regeneration zone will also react with carbon monoxide produced by the combustion of coke thereby yielding a further evolution of heat.
When CO combustion occurs in a relatively catalyst free zone of the regenerator, such as the region above a dense fluidized bed, the resulting high temperatures may lead to severe equipment damage. Such situations may be avoided if the CO combustion takes place in the presence of catalyst particles which act as a heat sink. Therefore, regenerators are generally designed to avoid the combination of free oxygen and carbon monoxide in regions that are relatively free of catalyst. Despite this the heat evolved from unintended CO combustion may raise the temperature of the catalyst to the point of causing thermal deactivation of the catalyst or may affect the process by limiting the amount of catalyst that can contact the feedstock. The problems of controlling catalyst and regenerator temperatures are exacerbated by the application of FCC processes to crack heavy feedstocks. With the increased coke producing tendencies of these heavy or residual feeds a complete regeneration of catalyst becomes more difficult due to the excessive heat evolution associated with coke and CO combustion.
Aside from excessive heat evolution the complete oxidation of CO also increases overall oxygen demands for the regeneration process. In many cases the high oxygen requirements for complete regeneration may exceed the limited capacity of the regenerator air blower or other equipment in the regenerator and flue gas section. Therefore, it is also desirable to reduce CO combustion so that a greater quantity of the available air supply is used to oxidize coke from the catalyst.